The present invention relates to a method for making a rare earth superconductive composite.
A superconductive material loses electrical resistance at temperatures at or below a critical temperature. Recently, a new class of rare earth-alkaline earth-copper oxide was discovered to be a superconductive material having a critical temperature above 77K, which is a boiling point of liquid nitrogen. This type of compound has an approximate unit formula of YBa.sub.2 Cu.sub.3 O.sub.7-x, where x is typically about 0, and x represents oxygen deficiency. Crystalline structure of this type of rare earth oxide superconductors have a perovskite structure with the oxygen deficiency.
Any superconductor can be classified into either type superconductor or type II superconductor. When the type I superconductor is exposed to magnetic field below a critical magnetic field, the superconductor shows complete diamagnetism due to the Meissner effect, and magnetic flux can not penetrate inside the superconductor. A supercurrent flows at the surface of the superconductor so as to cancel the external magnetic field. At the critical magnetic field the superconductor undergoes a transition from a superconductive phase to a non-superconductive phase, and magnetic flux begins to penetrate the inside the superconductor.
The type II superconductor exposed to magnetic field below the first magnetic field behaves like the type I superconductor, and magnetic flux can not penetrate inside the superconductor due to the Meissner effect. At the first critical magnetic field the superconductor undergoes a transition from a superconductive phase to a mixed phase, and magnetic flux begins to penetrate the inside the superconductor while maintaining superconductivity. At the second critical magnetic field the type II superconductor undergoes a phase transition from this mixed state to a non-superconductive state. The oxide superconductor belongs to type II superconductor.
When an electric current is applied to the type II superconductor in the mixed state, magnetic flux moves due to Lorenz force thereby showing electrical resistance. However, when pinning centers, which include, for example, diamagnetic particles, are dispersed in the superconductor, the magnetic flux penetrates though the pinning centers so that magnetic flux is prevented from moving.
Recently, active studies have been reported to obtain a rare earth oxide superconductor having fine particles dispersed therein acting as pinning centers.
Japanese Patent Application Laid-Open No. 4-224111 discloses a use of a platinum group element with a partial melt process so as to disperse fine particles of RE.sub.2 BaCuO.sub.5 (RE is Y, Gd, Dy, Ho, Er or Yb) in superconductive grains of REBa.sub.2 Cu.sub.3 O.sub.7-x. The fine particles of RE.sub.2 BaCuO.sub.5 work as pinning centers to prevent magnetic flux from moving in the resulting superconductive composite. The process includes the steps of: heating a green compact including REBa.sub.2 Cu.sub.3 O.sub.7-x and at least one element of Rh, Pt, Pd, Ru, Os, and Sc to a temperature higher than an incongruent melting temperature of REBa.sub.2 Cu.sub.3 O.sub.7-x to partially melt REBa.sub.2 Cu.sub.3 O.sub.7-x ; slowly cooling the resulting material to recrystallize REBa.sub.2 Cu.sub.3 O.sub.7-x from the melt. The presence of a platinum group element, such as platinum and rhodium, in the melt is associated with a mechanism to disperse fine particles of RE.sub.2 BaCuO.sub.5 in the slow cooling step.
Ogawa, Yoshida and Hirabayashi in ISTEC Journal, Vol. 4, No. 3, 1991, p. 30, describes a generally similar concept of a partial melting process in which a green compact includes a small amount of a platinum group element to make a rare earth superconductive composite in which fine particles of Y.sub.2 BaCuO.sub.5, acting as pinning centers, are dispersed in superconductive grains of REBa.sub.2 Cu.sub.3 O.sub.7-x.